3-D Sound and Spatial Audio

3-D Sound and Spatial Audio MUS_TECH 348

Multi-Loudspeaker Reproduction: Surround Sound

Some current issues: Multi-loudspeaker Reproduction • Can sound material be authored in a single format for headphones, near-field loudspeakers, and surround sound? • How should music be mixed for 5.1 reproduction?

Multi-loudspeaker Reproduction How would a general purpose system be designed? Encode 3D Decode 3D transmission

Multi-loudspeaker Reproduction Jot, et. al article compares and evaluates alternative systems attempting to use objective criteria, though not perceptual criteria: • Panning • HRTF techniques • Ambisonics

Multi-loudspeaker Reproduction What are the key issues? inN # of inputs? Encode 3D combined? in2 in1 transmission # of channels reproduction formats: hp 2 speaker 5.1 … Decode 3D

Multi-loudspeaker Reproduction What are the potential tradeoffs? • fidelity • timbre • direction • # of channels • listener freedom

Multi-loudspeaker Reproduction Ambisonics Gerzon “Ambisonics in Multichannel Broadcasting and Video” Originally conceived of as an alternative to quadraphonic sound (especially an alternative to stereo-encoded quad) Ambisonics is actually an encode method that is independent of the number of output channels and a decode method that is adaptable to reproduction with an arbitrary number of loudspeakers. Techniques were pioneered by Michael Gerzon, Mathematical Institute at Oxford, and P.E. Fellgett, University of Reading. Duane Cooper, University of Illinois, deserves some credit for establishing precedents.

Multi-loudspeaker Reproduction Ambisonics Ambisonic formats: B-Format - 4 channels with sum and differences (We focus on this) Originally conceived in connection with recording with the soundfield microphone.

Multi-loudspeaker Reproduction Ambisonics Ambisonic formats: UHJ - 4 channels with hierarchic encoding for scaled reproduction G-Format - no decoder

Multi-loudspeaker Reproduction Ambisonics • First-order ambisonic encoding • W = S Source Sound • X = S.x = S √2cos q cos ø Front-Back • Y = S.y = S √2sin q cos ø Left-Right • Z = S.z = S √2sin ø Elevation • Where q is azimuth and ø is elevation • Z is used for elevation, but when there is no elevated loudspeaker, it is omitted for a 3-channel 2D Ambisonics

Multi-loudspeaker Reproduction Ambisonic Encode w x(q,n) 4 - ch Mixer x in transmission y(q,n) y z(h) z

Multi-loudspeaker Reproduction Ambisonics • Second-order ambisonic encoding • Enables greater specificity in the spatial resolution • For horizontal plane add the following: • U = S cos (2q) cos ø • V = S sin (2q) cos ø

Multi-loudspeaker Reproduction Ambisonics • Ambisonic Decoder • For an N-channel first-order decoder with a regular loudspeaker geometry: • Si = gi S = 0.5 [ k0 W + k1 X cos qi + k1 Y sin qi ] • For large-space reproduction, k0 and k1 are the same: • k0 = k1 = sqrt( 8 / 3N) • where N is the number of loudspeakers • Other loudspeaker geometries can be calculated!

Multi-loudspeaker Reproduction Ambisonic Decode 1 w 2 x 3 y z N

Multi-loudspeaker Reproduction Ambisonics Soundfield rotations:

3-D Sound and Spatial Audio